Most porous formations contain clay minerals, which are crystalline in nature and have lattice-layer silicates and chain silicates. The lattice-layer silicates are formed of combinations of two basic building blocks, a silicone-oxygen tetrahedron, and an aluminum-oxygen-hydroxyl octahedron. These units are polymerized into sheets. Tetrahedral sheets are formed by sharing of corners, while octahedral sheets are formed by sharing of edges. There are two types of octahedral sheets: one in which every octahedral site is filled by a divalent ion and one in which two out of three sites are filled by trivalent ions. The first and second sheets are referred to as trioctahedral and dioctahedral sheets, respectively. The polymerization process can also be continued by hooking together tetrahedral and octahedral sheets to form a 1:1 composite layer. In the composite layer, the octahedral sheet could also be a dioctahedral one. Similarly, a 2:1 composite layer can also be formed by using two tetrahedral sheets to the central octahedral layer. At 2:1 composite layer could be formed of dioctahedral or trioctahedral sheets.
Clay surfaces of the most common clays have many negatively charged sites, which make them fresh-water sensitive. Previous studies have established that clay occur naturally as either pore-lining or pore-filing minerals. These clay minerals usually are surrounded by saline connate water layer. The cations (e.g., Na.sup.+, Ca.sup.++ etc.) from the saline water neutralizes the negative charges in clay minerals. The introduction of fresh water or less saline water into the formation, dilute the connate water and reduce its saline content. Because of this cation-charge deficiency around clay minerals, water molecules can easily invade in between clay platelets and results in swelling or dispersion. Therefore, charge deficiency in the minerals is an important quantity. It determines the forces holding the layers together. With mica group, these forces are relatively large. For smectites the forces are relatively small. The results is that interlayer cations can leave and enter the structure readily. In addition, water and organic molecules can enter and leave readily. Water tends to hydrate interlayer cations and result in a swelling of structure perpendicular to these layers. Organic materials also cause swelling.
Clay materials either were originally deposited during sedimentation, were formed later by the action of heat, pressure, and time on minerals already present, or were precipitated from fluids flowing through the matrix. The major components of clay are smectite, kaolinite, illite and mixed layer (i.e., illite-smectite). The two major mechanisms by which these minerals cause permeability damage are swelling and migration. In swelling, clay imbibes fresh water into its crystalline structure and subsequently increases in volume, plugging the pores in which it resides. Mixed layer and smectite are examples of swelling clays. In migration, clay minerals can be dispersed by contact with a foreign fluid or can be entrained by produced fluids and transported until a restriction is encountered (usually a pore throat), where the entrained particles bridge and restrict flow in the capillary. Kaolinite, illite, chlorite and mixed-layer are examples of migrating clays.
During drilling, if water-based drilling mud is used, mud filtrate will invade and damage the near-wellbore formation to some degree. During completion, the completion fluid can also invade and damage the near-wellbore formation. The cause of the formation damage can be explained by several possible factors including:
1. the invasion of drilling fluid causes clay minerals to swell and to constrict pore throats; this constriction causes a decrease in formation porosity and permeability, and an increase of the capillary effects. PA1 2. the invasion of water-based fluid also causes water blockage due relative-permeability effects (two-phase flow). PA1 1. dehydration of clay lattices, PA1 2. vaporization of any blocked water, mud filtrate, or other fluids, and/or PA1 3. destruction of clay structure.
The above-mentioned factors and other possible factors subsequently cause permeability reduction. Hydraulic-fracture treatments are often effective in by-passing the clay-related formation damage. However, these treatment techniques of clay-related formation damage, especially in horizontal wells, are difficult to perform and could be uneconomic. Therefore, there is a need in the petroleum industry for a new and improved method of treating clay-related formation damage.
In addition to the conventional acid and hydraulic-fracture treatments, several unconventional methods are disclosed in the literature. The following is a brief description of some of these disclosures.
U.S. Pat. No 4,844,169 presents a method of injecting non-reactive gas (i.e., nitrogen) into the formation at atmospheric temperature to fluidize the clays, including migratable fines, for their removal. Subsequently, an aqueous solution of soft water containing potassium chloride is proposed to be injected into the formation to cause a potassium-sodium cationic exchange within the swellable clays to reduce their swelling. In this method, temperature is kept low and clay structures are not altered. The fluidized clay particles can also block pore throat and subsequently, the treating fluid will be unable to contact the swelling clays. After low-temperature injections, chemical treatments may cause reswelling of the clays.
Canadian patent No. 915,573 discloses a method of treating the near-wellbore formation damage by contacting the formation with heated air or gas at a 121.degree. C. (250.degree. F.) temperature to cause partial dehydration of clays. Thereafter, the near-wellbore formation is treated with non-ionic vinyl pyrrolidone polymer to prevent reswelling of clays. In this 2-step method, the partial dehydration remedies the formation damage temporarily. However, subsequent chemical treatment may not be very effective because of the lack of good contact between the polymer solution and the formation.
Injection of aqueous solution of nitrogen at an elevated temperature of 260.degree. C. to 310.degree. C. (500.degree. F. to 590.degree. F.) to transform montmorillonite clays to more stable illitic-type clays was disclosed in U.S. Pat. No. 4,227,575. These illitic-type clays are less sensitive to fresh water. The transformation of montmorillonite clays to illitic-type clays are possible by this method, but the aqueous solution of nitrogen can also trigger swelling of other minerals (e.g. glauconite paloids).
The use of saturated and superheated steam at temperatures of 104.degree. C. to 871.degree. C. (220.degree. F. to 1600.degree. F.) and at pressures of 14.7 to 8000 psia was proposed in U.S. Pat. No. 3,847,222 to treat the near-wellbore formation damage. Subsequent to steam treatment, the injection of guanidine hydrochloride in methanol was shown to achieve better results. In this two-step process, the condensed steam will act as a source of fresh water and cause formation damage in the untreated regions.
The simultaneous injection of steam and vaporized hydrogen chloride to rectify clay-related formation damage is presented in U.S. Pat. No. 4,454,917. The purpose of steam is to clean the formation and the purpose of hydrogen chloride is to react with calcium and magnesium salts in the near-wellbore formation to form water-soluble chloride salts. In this process, the condensed steam is also a source of fresh water and could cause formation damage due to reswelling of clay minerals.
Another preventive technique disclosed in Canadian patent No. 1,282,685 is the removal of precursor ions from the injection water using reverse osmosis before injection into the formation. The removal of precursor ions will reduce precipitation in the formation and subsequently reduce the chances of formation damage. In this technique, the removal of precursor ions may not necessarily prevent the swelling and/or migration of clay materials in the formation.